1. Field of the Invention
A major goal in the pharmacological arts has been the development of methods and compositions to facilitate the specific delivery of therapeutic and other agents to the appropriate cells and tissues that would benefit from such treatment, and the avoidance of the general physiological effects of the inappropriate delivery of such agents to other cells or tissues of the body. One common example of the need for such specificity is in the field of antiproliferative agent therapy, in which the amount of a variety of antiproliferative agents to be safely administered topically or locally to a patient is limited by their systemic cytotoxic effects.
In addition, it is recognized in the medical arts that certain subcellular organelles are the sites of pharmacological action of certain drugs or are involved in the biological response to certain stimuli. Specific delivery of diagnostic or therapeutic compounds to such intracellular organelles is thus desireable to increase the specificity and effectiveness of such clinical diagnostic or therapeutic techniques.
A. Drug Targeting
It is desirable to increase the efficiency and specificity of administration of a therapeutic agent to the cells of the relevant tissues in a variety of pathological states. This is particularly important as relates to antiproliferative agents. Such agents typically have pleiotropic antibiotic and cytotoxic effects that damage or destroy uninvolved cells and tissues as well as cells and tissues comprising the pathological site. Thus, an efficient delivery system which would enable the delivery of such drugs specifically to the diseased or affected tissues cells would increase the efficacy of treatment and reduce the associated "side effects" of such drug treatments, and also serve to reduce morbidity and mortality associated with clinical administration of such drugs.
Numerous methods for enhancing the cytotoxic activity and the specificity of drug action have been proposed. One method, receptor targeting, involves linking the therapeutic agent to a ligand which has an affinity for a receptor expressed on the desired target cell surface. Using this approach, a drug is intended to adhere to the target cell following formation of a ligand-receptor complex on the cell surface. Entry into the cell could then follow as the result of internalization of ligand-receptor complexes. Following internalization, the drug may then exert its therapeutic effects directly on the cell.
One limitation of the receptor targeting approach lies in the act that there are only a finite number of receptors on the surface of target cells. It has been estimated that the maximum number of receptors on a cell is approximately one million (Darnell et al., 1986, Molecular Cell Biology, 2d ed., W. H. Freeman: New York, 1990). This estimate predicts that there may be a maximum one million drug-conjugated ligand-receptor complexes on any given cell. Since not all of the ligand-receptor complexes may be internalized, and any given ligand-receptor system may express many-fold fewer receptors on a given cell surface, the efficacy of intracellular drug delivery using this approach is uncertain. Other known intracellular ligand-receptor complexes (such as the steroid hormone receptor) express as few as ten thousand hormone molecules per cell. Id. Thus, the ligand-receptor approach is plagued by a number of biological limitations.
Other methods of delivering therapeutic agents at concentrations higher than those achievable through the receptor targeting process include the use of lipid conjugates that have selective affinities for specific biological membranes. These methods have met with little success. (see, for example, Remy et al., 1962, J. Org. Chem. 27: 2491-2500; Mukhergee & Heidelberger, 1962, Cancer Res. 22: 815-22: Brewster et al., 1985, J. Pharm. Sci. 77: 981-985).
Liposomes have also been used to attempt cell targeting. Rahman et al., 1982, Life Sci. 31. 2061-71 found that liposomes which contained galactolipid as part of the lipid appeared to have a higher affinity for parenchymal cells than liposomes which lacked galactolipid. To date, however, efficient or specific drug delivery has not been predictably achieved using drug-encapsulted liposomes. There remains a need for the development of a cell- or organelle-targeting drug delivery system.
In response to the deficiencies encountered with receptor targeting, investigators have looked for other methods of delivering therapeutic agents at concentrations higher than those achievable through the receptor targeting process. Experiments suggested that lipids have selective affinities for specific biological membranes.
The selective association of certain lipids with specific biological membranes provided a possible avenue of drug targeting. In light of this possibility, researchers have attempted to target drugs by conjugating them with cholesterol. Unfortunately, these attempts have met with disappointing results (see, Remy et al., 1962, J. Org. Chem. 27:2491-2500; Mukhergee and Heidelberger, 1962, Cancer Res. 22: 815-22; Brewster et al., 19xx, J. Pharm. Sci. 77: 981-985, have had some success with carrying estradiol to the brain using pyridinium salts as carriers.
Another attempt at cell targeting through the use of lipids was made by Rahman et al., 1982, Life Sci. 31: 2061-71. These investigators found that liposomes which contained galactolipid as part of the lipid appeared to have a higher affinity for parenchymal cells than liposomes which lacked galactolipid. These researchers suggested that this finding might have utility in drug targeting.
An additional challenge in designing an appropriate drug delivery scheme is to include within the drug conjugate a functionality which could either accelerate or reduce the rate at which the drug is released upon arrival at the desired site. Such a functionality would be especially valuable if it allowed differential rates of drug release.